Formation of electrode tip by electric current heating



May 25, 1954 L. E CISNE FORMATION OF ELECTRODE TIP BY ELECTRIC CURRENTHEATING Filed Aug. 18, 1951 I I I4 lNl ENTOR L. E. c/s/vE; 6 WM A TTOR/VE Y Patented May 25, 1954 FORMATION OF ELECTRODE TIP BY ELECTRICCURRENT HEATING Luther E. Cisne, Glen Ellyn, Ill., assignor to BellTelephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application August 18, 1951,Serial No. 242,565

1 Claim.

This invention is concerned with the shaping of the ends of wires orfilaments and more particularly to making spherical terminations on suchwires or filaments, which are to be used, for example, as electrodes.Wires thus terminated are suitable for use as the contact wires incrystal rectifiers of the point contact type and for similar devices.

It is desirable in the manufacture of point contact wires for crystalrectifiers and like devices to closely control the area of contactbetween the wire and the crystal surface. For example, in point contactdevices of this type, which are to be used at very high frequencies, anoptimum contact area capable of. exhibiting at the same time suitableresistance and capacitance characteristics is of considerableimportance. 1

Contact wires falling in the category under discussion are usually madewith a spring section, the function of which is to apply a suitablecontact pressure. The spring section may take various shapes, e. g.,helical, C-shape, S-shape, etc. The S-shape is commonly used and mayconveniently form the basis for a description of some of the details ofthis invention.

The area of contact between the wire tip or point and the crystalsurface, for example, may be considered as a function of several.factors, one of which is point shape. With the usual sharp point, aconsiderable force is expanded.

in the inelastic deformation of the point to produce a suitable area ofcontact. A spherical point on the other hand would have a greater and amore predictable range of elastic deforms.

tion. Such a point would allow the realization of a contact area ofsuitable proportions with a much smaller total force than that requiredwith sharper points. This would result among other things, in a lowerunit contact pressure and consequently less strain at or near thesurface of the crystal.

The foregoing and other desirable features may be realized by makingspherical points or termi nations in accordance with the method of thisinvention.

An object of this invention is to enable the ready fabrication of anelectrode or contact wire having a spherical point, free from surfaceirregularities and composed of fine grain homogeneous material.

A feature of this invention resides in controllably melting a smallportion of a wire by Joulean heat, whereby a spherical. surface ofrelatively fine grain material is formed.

A further feature of this invention pertains to cooling portions of awire adjacent to the heated portion to confine the melting to thedesired zone and to aid in controlling the shape of the point made.

Another feature of this invention resides in 2 controlling the speed ofheating to aid in determining the size and configuration of thespherical portion of the wire.

Other and further objects and features of this invention will appearmore fully and clearly from the following description of exemplaryembodiments thereof taken in connection with the appended drawings inwhich:

Fig. 1 is a perspective view of an apparatus suitable for carrying outthe method of this invention, the heating circuit being of thedirectcurrent type;

Fig. 2 is a plan view of like apparatus with an alternating-currentcircuit illustrated;

Figs. 3 to 6, inclusive, show various point configurations obtainablewith this method, and

'7 is an elevational view of the apparatus provided with a cover toconfine the heating zone to a desired ambient atmosphere.

As illustrated in Figs. 1, 2 and 7, a wire it of tungsten, Phosphorbronze, platinum or other suitable material, may serve as the basis formaking two spherical points. The wire is, in this embodiment of theinvention, clamped between two pairs of conductive blocks i ill-l lb and2a-- lib by clamps i3 and H5, respectively.

An important function of the pairs of blocks ila-i lb and l2a-l2b is toserve as heat sinks or storage means to aid in confining the heating,later to be described, to a limited zone of the wire it intermediatethese two heat sinks, The heat sinks may for convenience be designatedrespectively as I! and it.

In order that tension may be applied to the wire iii, the clamp i3 issecured by the fas ening means E5 to the base it. A weight ll secured bya cord 13 to the clamp Hi passes over pulley is to apply tension to thewire. The size of the weight ll may be chosen to give a de sired tensionon wire it or if different values of tension are needed a plurality ofgraded weights may be used. Other means such as a spring of suitableproportions, motor means, etc, may be employed instead of the weight iito apply the proper tension to the wire ill.

Electric power for heating the wire it may be introduced by variousmeans such as the binding posts 20 and ti secured respectivelyto theheat sinks ll and I2. Either direct or alternating current may be used.

A direct current may be obtained from a battery 22 connected through aswitch 23 and an adjustable resistor 24 for limiting the current to asuitable value, as illustrated in Fig. 1. If an alternating current isto be used, means comprising a transformer 25, a switch 26,. and anadjustable impedance 2'! may be connected between the binding posts 202land an alternatingcurrent power line, as illustrated in Fig. 2. Othersuitable current supply means may obviously be used, one such being ameans for applying a condenser discharge to the Wire.

In those cases where it is desirable to exclude oxygen from the wire toavoid oxidation thereof during heating, or to control the pressure, orother characteristics of the ambient atmosphere, an enclosure may beprovided. This may take various forms one of which may be a glass orlike cover 35 as illustrated in Fig. 7. If a nonoxidizinglighter-than-air gas, such as hydrogen, is to be used, the cover maysimply rest on the base E5. The hydrogen may be introduced through theopening that accommodates cord 18 or by any other convenient means. Ifanother gas requiring a tighter enclosure or if pressure higher or lowerthan atmospheric is used, the enclosure may be by any convenient e2pedient.

The formation of points on the wire IE3 is accomplished by passingsuflicient current through the wire to heat its intermediate portion tothe melting point, whereby the wire is severed and spherical ends formedthereon due to the effect of surface tension forces in the wire.

Figs. 3 to 6, inclusive, illustrate some point configurations which maybe obtained. The points of Fig. 3 are of substantially the same radiusas the wire. This illustrates what may be called an intermediatesituation. That is, with a given size of wire-and spacing of heat sinks,the point radius is a function of the speed of heating. A moderate speedwill give a point as in Fig. 3. If the heating speed is higher, lessmaterial is melted and a large radius, relatively flat point as at Fig.l, is formed. At a relatively low heating speed more material is meltedand the point takes the appearance of Fig. 5 with a radius greater thanthat of the wire and a ball point. In other words, the points of Fig. 3are minimum radius points. The variations in point contour thus fardescribed assume substantially no deformation of the wire due to thetension thereon.

If a point of less radius than that of the wire is desired, it may beobtained by relatively slow speed heating and relatively high tension.Under these conditions the wire, while in a plastic condition prior tomelting, may be reduced or necked by the tension and may break beforemelting. In

this case the arc across the break will melt the ends of the wire andproduce spherical surfaces of smaller radius than that of the wire asillustrated in Fig. 6. In order to have the arc of sufficient durationto melt the wire ends, a directcurrent source should be used, since analternatingwurrent arc would extinguish too quickly to accomplish thepurpose.

If the minimum radius required is that of the wire, as illustrated inFig. 3, the tension on the wire should be relatively small along with amoderate heating rate as previously noted. It has been found that anoptimum heating rate for producing the Fig. 3 type of point may be foundfor a fair range of tensions. For example, tensions of from three poundsto less than onehalf pound were used successfully with tungsten wire.005 inch in diameter.

Spherical points may be made with substantially no tension on the wire,although low tension appears to be somewhat more advantageous. It ispossible to make spherical points by clamping separate wires in the twoheat sinks, bringing their ends together, applying direct current to thejoined wires and then pulling the wires apart to draw an are which meltsthe wire ends.

This is somewhat like what is done in forming the points of Fig. 6without the initial reduction in diameter and parting of the wire duringthe plastic stage.

If rectifier contact wires having an e-shaped or other convolutedportion are to be pointed by this method, the blocks [Ia and lib may beprovided with mating grooves of proper configuration to locate the wireso that the point will be formed at the proper place. Grooves of thistype may also be provided in blocks H0. and i212 so that two contactwires of given configuration may be pointed at the same time.

The heat sinks may be of copper, brass or other material of relativelyhigh thermal conductivity. With .005 inch diameter wire, blocks of brassor of copper .625 inch square and .250 inch thick have been used givinga heat sink .625 inch by .625 inch by .500 inch. Heat sinks about thevolume of the foregoing are also satisfactory. The spacing between theheat sinks for a .005 inch diameter wire may be from .075 to .150 inch,the greater separation giving larger balls at the end of the wire. Withboth heat sinks of about the same heat absorbing ability, the wire issevered at a point substantially equidistant from each sink.

The size of the balls on the wire ends may be controlled by the pressureof the ambient atmosphere, being large for a vacuum, medium foratmospheric pressure, and small for pressures above atmospheric. Theeffect of this environmental control is correlated with the other heatcontrolling factors to secure a desired over-all effect.

The fine drawn wires of tungsten and other suitable materials used forthe point contacts in rectifiers and like devices usually have a grainstructure that is coarse relative to their diameters. As a consequenceof this grain structure it is necessary when making points by etching orgrinding, to be extremely careful to avoid a rough surface. The meltingof the present invention fines the grain of the metal and makes for avery j smooth as well as a spherical point or tip.

The term spherical as herein employed denotes a surface that is aportion of a sphere and is not intended to cover a complete sphere.

Although this invention been described by means of illustrativeembodiments thereof it is not intended that these be taken as limitingthe scope of the invention.

What is claimed is:

Apparatus for making spherical termination a wire filament by theoperation of heat and surface tension. fcrces therein, that comprises abase, spaced U-shaped clamps supported by said base with their open endsrelation, means for anchoring one of cl maps to said base, means forapplying a measured force to the other of clamps to eve aid clamp awayfrom the anchored clamp. a of removable massive heat absorbing blockssecured in each of said clamps for gripping spaced portions of thefilament, and a regulatable source of electric power connected acrosssaid blocks for heating the portion of the filament intermediate saidblocks.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 363,550 Sedgwick May 24, 1887 396,014 Thomson Jan. 8, 18892,399,466 Carlson Apr. 30, 1946

